There are known methods to harvest the heat below the surface to drive motors and generate electricity or perform other work. One type of geothermal system involves producing hot water and/or steam already present in underground formations. Another type of geothermal system involves introducing fluids directly into the underground formations to absorb the heat and recovering the fluids, along with the absorbed heat. Typical disclosures of these types of geothermal systems are provided by U.S. Pat. Nos. 4,082,140, 4,201,060, and 4,357,802, as well as in commercial operations in California and Iceland.
Other methods of harvesting this heat includes geothermal systems that pump a working fluid, such as water, through a casing in a bore hole that extends into a hot section of the earth's crust. The heat in the earth turns the working fluid into steam, which is produced at the surface, and then condensed and purified to be pumped back into the casing. Typical disclosures of this type are found in U.S. Pat. Nos. 3,470,943, 4,085,795, 5,072,783, 5,515,679, and 6,301,894.
This type of geothermal system has certain advantages over producing heated formation fluids directly or introducing fluids into the formations to be produced, such as minimizing problems of subsidence or seismicity in the rocks adjacent the geothermal wells and disposing produced fluids. While it is an improvement, this type of geothermal system still suffers from certain disadvantages. For instance, these geothermal systems often perforate the casing to facilitate the heat exchange between the formation and the working fluid. As such, additional water from another source such as well, lake, or municipal is often needed to be pumped through the casing to recover the heat below. Further, due to the perforations of the casing introducing particulates from the surrounding environment to the heated fluid, this type of geothermal system also often require a purification system to process the heated fluid before it can be circulated again.